1. Field of Invention
This invention relates to high durability ceramic tools comprising silicon nitride.
2. Description of the Prior Art
As ceramic tools for rough cutting castings at a speed as high as 100 m/min or more, tools produced by sintering through hot-pressing or hot isostatic pressing process (HIP process) have been primarily used. However, these tools tend to chip or crack during use which, in the case of cutting using an automatic cutting machine, has been the primary obstacle to operating efficiency. Also, the life of the tools is dominated by the chipping or cracking and, even when tools are worn only to a slight depth, they often have to be replaced due to chips or cracks.
As a result of intensive investigations to overcome the above defects, the inventors have discovered silicon nitrides having higher impact resistance, heat shock resistance, and durability than Al.sub.2 O.sub.3 based materials.
Ceramics mainly comprising silicon nitride have a strong covalent bonding character and are difficult to sinter, and many studies have been made directed to their use as engineering ceramics such as in gas turbines. However, few have been applied as cutting tools.
The inventors have examined the characteristics of materials as cutting tools using MgO and stabilized ZrO.sub.2 as sintering aids and, at last, have successfully overcome the defects of chipping and cracking during use, which have been the fatal defects of Al.sub.2 O.sub.3 based ceramic tools.